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Using tesserract with python

Using tesserract with python

First, we’ll learn how to install the pytesseract package so that we can access Tesseract via the Python programming language.

Next, we’ll develop a simple Python script to load an image, binarize it, and pass it through the Tesseract OCR system.

Finally, we’ll test our OCR pipeline on some example images and review the results.

Installing the Tesseract + Python “bindings”
Let’s begin by getting pytesseract installed. To install pytesseract we’ll take advantage of pip .

If you’re using a virtual environment (which I highly recommend so that you can separate different projects), use the workon command followed by the appropriate virtual environment name. In this case, our virtualenv is named cv .

Using Tesseract OCR with Python
$ workon cv
Next let’s install Pillow, a more Python-friendly port of PIL (a dependency) followed by pytesseract .

Using Tesseract OCR with Python
$ pip install pillow
$ pip install pytesseract
Note: pytesseract does not provide true Python bindings. Rather, it simply provides an interface to the tesseract binary. If you take a look at the project on GitHub you’ll see that the library is writing the image to a temporary file on disk followed by calling the tesseract binary on the file and capturing the resulting output. This is definitely a bit hackish, but it gets the job done for us.

Let’s move forward by reviewing some code that segments the foreground text from the background and then makes use of our freshly installed pytesseract .

Applying OCR with Tesseract and Python
Let’s begin by creating a new file named ocr.py :

Using Tesseract OCR with Python
# import the necessary packages
from PIL import Image
import pytesseract
import argparse
import cv2
import os
# construct the argument parse and parse the arguments
ap = argparse.ArgumentParser()
ap.add_argument(“-i”, “–image”, required=True,
help=”path to input image to be OCR’d”)
ap.add_argument(“-p”, “–preprocess”, type=str, default=”thresh”,
help=”type of preprocessing to be done”)
args = vars(ap.parse_args())
Lines 2-6 handle our imports. The Image class is required so that we can load our input image from disk in PIL format, a requirement when using pytesseract .

Our command line arguments are parsed on Lines 9-14. We have two command line arguments:

–image : The path to the image we’re sending through the OCR system.
–preprocess : The preprocessing method. This switch is optional and for this tutorial and can accept two values: thresh (threshold) or blur .
Next we’ll load the image, binarize it, and write it to disk.

Using Tesseract OCR with Python
# load the example image and convert it to grayscale
image = cv2.imread(args[“image”])
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
# check to see if we should apply thresholding to preprocess the
# image
if args[“preprocess”] == “thresh”:
gray = cv2.threshold(gray, 0, 255,
# make a check to see if median blurring should be done to remove
# noise
elif args[“preprocess”] == “blur”:
gray = cv2.medianBlur(gray, 3)
# write the grayscale image to disk as a temporary file so we can
# apply OCR to it
filename = “{}.png”.format(os.getpid())
cv2.imwrite(filename, gray)
First, we load –image from disk into memory (Line 17) followed by converting it to grayscale (Line 18).

Next, depending on the pre-processing method specified by our command line argument, we will either threshold or blur the image. This is where you would want to add more advanced pre-processing methods (depending on your specific application of OCR) which are beyond the scope of this blog post.

The if statement and body on Lines 22-24 perform a threshold in order to segment the foreground from the background. We do this using both cv2.THRESH_BINARY and cv2.THRESH_OTSU flags. For details on Otsu’s method, see “Otsu’s Binarization” in the official OpenCV documentation.

We will see later in the results section that this thresholding method can be useful to read dark text that is overlaid upon gray shapes.

Alternatively, a blurring method may be applied. Lines 28-29 perform a median blur when the –preprocess flag is set to blur . Applying a median blur can help reduce salt and pepper noise, again making it easier for Tesseract to correctly OCR the image.

After pre-processing the image, we use os.getpid to derive a temporary image filename based on the process ID of our Python script (Line 33).

The final step before using pytesseract for OCR is to write the pre-processed image, gray , to disk saving it with the filename from above (Line 34).

We can finally apply OCR to our image using the Tesseract Python “bindings”:

Using Tesseract OCR with Python
# load the image as a PIL/Pillow image, apply OCR, and then delete
# the temporary file
text = pytesseract.image_to_string(Image.open(filename))
# show the output images
cv2.imshow(“Image”, image)
cv2.imshow(“Output”, gray)
Using pytesseract.image_to_string on Line 38 we convert the contents of the image into our desired string, text . Notice that we passed a reference to the temporary image file residing on disk.

This is followed by some cleanup on Line 39 where we delete the temporary file.

Line 40 is where we print text to the terminal. In your own applications, you may wish to do some additional processing here such as spellchecking for OCR errors or Natural Language Processing rather than simply printing it to the console as we’ve done in this tutorial.

Finally, Lines 43 and 44 handle displaying the original image and pre-processed image on the screen in separate windows. The cv2.waitKey(0) on Line 34 indicates that we should wait until a key on the keyboard is pressed before exiting the script.

Let’s see our handywork in action.

Tesseract OCR and Python results
Now that ocr.py has been created, it’s time to apply Python + Tesseract to perform OCR on some example input images.

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